Discovered by Galileo in 1610, bright, yellow Europa is the smallest of JupiterΓÇÖs moons. Its surface is very smooth. The dark lines in this pictureΓÇötaken by the Voyager spacecraft from a distance of 19 million kilometersΓÇöare superficial cracks. Although EuropaΓÇÖs crust is thought to be icy, its interior is hot. One theory suggests that EuropaΓÇÖs interior used to be hot enough to melt the crust, but over time the surface cooled and became hard and smooth.
